What is a GBB (Gas Blow-Back)?

The Gas Blow-Back is a relatively new phenomenon.  Designed to operate in the most realistic manner, they are great fun to shoot.  These replicas are typically semi-automatic pistols like the Colt 45 or the Glock and are powered by a refrigerant gas.  The power level of the GBB's shot is roughly the same as that of a stock AEG, but the shooting experience is much more fun.

What makes a GBB worth buying?

Airsoft guns are designed to be as realistic as possible, both in looks and operation.  GBBs absolutely excel here as this was part of the brief for the original designs.

The main benefits of the GBB are...

• Reliability
  GBBs tend to be reliable if they have a simple design (e.g. KSC Glock pistols).  These models can provide thousands of shots without jamming or breaking.  More complex replicas such as some SMGs regularly suffer from jams and breakages, although some users report that these are very sturdy guns.
  GBBs can become unreliable if used with the wrong pressure gas.  Gas that is too powerful can cause random breakages and gas that is too weak can cause jams.  The ambient temperature also affects the pressure and performance of gasses.  With practice and the right information, finding the right gas is fairly simple.
  Fairly regular cleaning and lubrication is required in order to protect the rubber seals and moving parts.  This is very easy though, as most GBBs are 'field-strippable' (i.e. they can be taken apart quickly without the need for tools).
  Gas Blow-Backs are generally considered to be less reliable than AEGs.

• Accuracy
  A well made GBB that has been well maintained can be very accurate - just as accurate as an AEG.  They are powerful compared to most Non Blow-Back and 'Springer' guns, so their effective range is quite good.
  Gas Blow-Backs are generally less accurate than AEGs at longer distances though.  Part of this is down to pressure differences and the cooling effect from the gasses used.  This will affect the consistency of shots, reducing overall accuracy, especially at a distance.

• Compact
  GBBs do not need to hold batteries, gears, pistons or large main-springs.  This means that they can be small.  One of the most common GBB pistols sold in the UK is the KSC Glock 17.  This pistol is a 1:1 replica of the real-steel Glock 17 and holds around 25 BBs.  These are an excellent backup to have in a skirmish and a lot of fun to use when shooting at targets.

• Light-Weight
  GBBs are generally made of plastic (occasionally light-weight metal alloy), making the pistol itself is fairly light.  The Magazine has thick metal walls, strong enough to hold the gas, so this can be quite heavy.  Overall, the replica is lighter than the real version and very easy to handle.

• Effective
  The replica uses gas to reload which has several major benefits...
          1. Reloading is instant - no delay.  This means BBs can be shot as fast as you can
              pull the trigger.  Even faster shots can be fired if the GBB has a full-automatic
              option.
          2. No reloading energy needs to be taken from the trigger-pull, allowing a clean
              crisp, easy pull.
          3. The sudden release of gas creates a considerable 'pop', giving the GBB a more
              realistic sound.  With a full-automatic option, the GBB makes a great rat-a-tat-tat!

• Upgradeable
  Quite a few upgrades are available for Gas Blow-Back replicas, but to be honest, these upgrades really should be part of the gun when you buy it.  The majority of upgrades are things like stronger internal parts, better springs and more capable valves.  Many GBB users buy these upgrade parts simply because the standard parts broke.
  However, some parts can be useful little add-ons, like tighter barrels, different slides and recoil enhancers.  All of these can help to put a smile on the face of any GBB owner.

How does a GBB work?

The overlay diagram below shows the basic structure of the GBB's insides.

Figure 1  -  The GBB's Internals

The Gas Blow-Back is a three dimensional machine and is difficult to understand simply from a diagram.  If you have a GBB of your own, you should be able to take it apart enough to see most of the components shown here.

The only notable component left out of these diagrams is the 'rocket-valve'.  This is found in most modern Gas Blow-Back replicas and sits just inside the blow-back cylinder (D above), behind the back of the air nozzle (the much thinner part of the cylinder facing forward).  It is a small valve that improves the efficiency of the mechanism by cutting off gas flow to the barrel when there is no BB inside.  It has been left out of these diagrams for simplicity.  There are many GBBs that operate perfectly without rocket-valves.

The GBB uses refrigerant gas as a power source.  It is shown as liquid in the diagram as it is actually in liquid form while stored in the magazine.  This gas is not as cost-effective as the AEG's battery pack, but it's a lot more realistic in operation.  Different gasses can be used to supply different pressures, as required by the weather conditions, but the effect of all of them is pretty much the same...

1.  The GBB at rest, before the first shot is fired. The GBB's hammer is actually not the cosmetic hammer that you can pull back with your thumb.  It is behind the magazine inside the grip of the piston (see 'B' on the diagram above).  This hammer is under tension and ready to be released, there is gas in the magazine and a BB in the breech.  This GBB is ready to be fired.
2. The trigger is pulled. This releases the hammer which strikes the firing valve (A in the diagram above) on the back of the magazine. Gas is immediately released upward out of the top of the magazine (J) and directly in to the blow-back cylinder (D).  The gas keeps flowing, forcing the BB fast down the barrel, as the building pressure in the cylinder starts the blow-back reloading mechanism.
3. The blow-back reloading mechanism operates. The pressure inside the cylinder pushes the blow-back ram (C) out of the back, taking the whole top-slide of the gun (E) as it goes. As the slide moves back, the hammer (B) is removed from the firing valve (A) and locked back ready for the next shot.  This closes the valve and halts the flow of gas from the magazine.
4. The pressure discipates. As the gas disappears from the blow-back cylinder, the cylinder itself is pulled back to its usual rest position over the ram by a small spring. At this point, the BB has already left the barrel and the gas is no longer powering the reloading mechanism.
5. The slide starts moving forward. At the start of this whole process, the top-slide was held in position (forward) by a spring.  When the gas initially forced the slide back, it pushed against this spring.  Now that the gas pressure has gone, this spring takes over again and brings the slide forward, back to its rest position. As the slide moves forward, the the air nozzle (the thinner front part of the single-piece blow-back cylinder), scoops a BB from the top of the magazine and starts pushing it forward, in to the breech.
6. The slide is reset. The slide hits its foremost point, the scooped BB is pushed firmly in to the breech, and all the other BBs in the magazine have been pushed up one notch to make up the gap.
7. That's it!  You can let go of the trigger now. The GBB is now in the same state as in step 1 and is ready for another shot.

This is how the most common Gas Blow-Back replicas work, but there are many variations on the same theme.  The most notable difference is when using an 'open-bolt' mechanism, as used by some airsoft SMGs.  This mechanism works exactly the same way as the mechanism shown above, except the gun's 'at-rest' position is position 4 above, not position 1.  As the trigger is pulled, the slide drops forward under the power of the slide's spring, the shot is fired and the slide is blown back, where it is held in place, ready for the next shot.  These 'open-bolt' mechanisms generally provide lower accuracy than the 'closed-bolt' method depicted in the diagrams above.

How good is it really?